drm/debugfs: drop unneeded DEBUG_FS guard

The Makefile enables/disables the file compilation depending on
CONFIG_DEBUG_FS.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231223183301.78332-1-dario.binacchi@amarulasolutions.com